Process for preparing perfluorinated vinyl ethers

ABSTRACT

Perfluorinated vinyl ethers are prepared by reacting a fluorinated carboxylic acid fluoride with hexafluoropropylene epoxide in the presence of a catalyst and thereafter pyrolyzing the carboxylic acid fluoride containing ether linkages, optionally after having converted it into a salt of a monovalent metal.

This is a division, of application Ser. No. 677,513, filed Apr. 16,1976, now U.S. Pat. No. 4,013,689.

Perfluorinated compounds are distinguished by their low chemicalreactivity and their thermal resistance. For this reason representativesof this type are used as heat transferring agents, dielectric agents,lubricants and solvents.

German Patent No. 1,162,829 discloses a process for preparingperfluorinated vinyl ethers, which comprises reacting a fluorinatedcarboxylic acid fluoride with hexafluoropropylene epoxide in thepresence of a catalyst and thereafter pyrolyzing the carboxylic acidfluoride containing ether linkages optionally after having converted itinto a salt of a monovalent metal.

It has now been found that also acid fluorides of the formula ##STR1##wherein n is 1 or 0, MAY BE USED AS CARBOXYLIC ACID FLUORIDES CONTAININGETHER LINKAGES FOR THIS PURPOSE.

The present invention consequently provides a process for preparingperfluorinated vinyl ethers of the formula ##STR2## wherein n is 0 or 1,WHICH COMPRISES HYDROLYZING AN ACID FLUORIDE OF THE FORMULA ##STR3## toyield the corresponding perfluorocarboxylic acid, neutralizing the acidobtained and heating the salt of the carboxylic acid obtained atneutralization to a temperature above 150° C.

The present invention moreover provides a process for the preparation ofperfluorinated vinyl ethers of formula I, which comprises heating anacid fluoride of the formula ##STR4## wherein n is 1 or 0, totemperature in the range of from 200 to 400° C; the pyrolysis of theacid fluoride is advantageously performed at a temperature of from 300°to 350° C.

The pyrolysis of the acid fluoride may be facilitated by catalysis onmetal oxides according to the process of U.S. Pat. No. 3,321,532.

The present invention finally provides the novel perfluoronated vinylethers of the formula ##STR5## wherein n is 1 or 0.

Transferring of the acid fluoride of formula II into a salt of amonovalent metal may be performed without difficulty by one skilled inthe art. The hydrogenolysis of the acid fluoride is carriedadvantageously with water containing about stoichiometrical quantitiesof a basic substance in a dissolved state. It is also possible tohydrolyze with pure water and to neutralize subsequently the acidsolution containing fluorides with basic substances. The salts areisolated thereafter, for example by evaporation of the aqueous solution.Suitable bases are especially compounds of alcaline metals, for examplehydroxydes or carbonates. The sodium and potassium salts are usedpreferably because of their quick and smooth decomposition. A removal ofthe inorganic fluoride formed simultaneously is not required generally.

Prior to its decomposition the carboxylic acid salt obtained must becarefully dried. The decomposition is performed in a closed apparatus,for example in a rotation evaporator, by heating to temperatures above150° C, generally of from 160 to 300° C, preferably of from 190 to 230°C, most suitably in vacuo, because the cleavage products in this caseonly remain for a short time in the pyrolysis zone. It is operatedpreferably under a pressure of less than 10 torrs, especially of from0.1 to 5 torrs. The vinyl ether formed of formula I is distilled off andmay be collected in cooled receivers and then be further purified,preferably by fractionated distillation.

Decomposition may also be performed under atmospheric pressure, wherebyby-products may easily be formed, for example products containinghydrogen, if the starting product is not absolutely dry.

The vinyl ethers obtained are colorless liquids and may be used in awide field of application as intermediates. They may be converted intosaturated fluorinated ethers, for example by addition of elementaryfluorine. The perfluoroethoxy derivatives obtained are chemically inertto a large extent. Perfluoro-[3.6-dimethyl-2-ethoxydioxane-1.4] forexample, may be used as lubricant, sealing liquid, heat transferringagent, insulating liquid or hydraulic liquid. The vinyl ethers may alsobe used as monomers for copolymers.

The vinyl ethers may especially react with perfluorinated acid fluoridesaccording to the equation ##STR6## wherein Rf means a perfluorinatedalkyl or polyoxalkyl radical, yielding saturated perfluorinated carbonylethers. This possibility is especially important for perfluorinatedpolyether carboxylic acid fluorides formed during the oligomerization orpolymerization of hexafluoropropene oxide, because the reactive terminalgroups of these polymers are masked thereby. In this process there areobtained thermally and chemically stable products having a molar weightincreased by the molar weight of the vinyl ether in comparison with thatof the starting compound. In ths way the complicated elimination of theCOF groups by means of elementary fluorine according to U.S. Pat. No.3,242,218 may be dispensed with. The products obtained when using theperfluorinated vinyl ethers according to the invention are neverthelessresistant towards and oxidants. The starting products of the processaccording to the invention (acid fluorides) are readily aceesible asthere is only obtained a small variety of oligomers during theirpreparation from hexafluoropropene oxide, oligomers only differing fromone another by the number of the added units of hexafluoropropene oxide.For this reason it is possible to prepare homogeneous vinyl ethers withvery good yields (calculated on hexafluoropropene oxide).

The following examples illustrate the invention:

EXAMPLE 1 Perfluoro-3,6-dimethyl-1,4-dioxanyl-2-vinyl ethers

2800 g of a mixture of hexafluoropropene oxide and hexafluoropropene (inweight ratio of 65:35) were introduced into a solution of 600 ml ofdiethylene glycol dimethyl ether and 600 g of PO[N(CH₃)_(2]3), in athree neck flask provided with an intensive condenser, a stirrer and athermometer for low temperatures, at a temperature from -40 to -30° C.while stirring continuously, at a rate of 40 l/hour (measured undernormal conditions).

Stirring was then continued for 5 hours at the above mentionedtemperature. Hexafluoropropene and the excess of epoxide then removed byheating slowly to 0° C. and the two phase mixture was separated in aseparating funnel. The lower phase (1742 g) was washed with 600 ml ofacetonitrile and yielded 1554 g of product mixture. 1142 g of asubstance boiling at a temperature of from 115 to 118° C. and 194 g of asecond fraction boiling at a temperature of from 118 to 170° C. wereobtained from the latter mixture by fractionated distillation.Elementary, infra-red and a NMR-spectroscopic analyses revealed theformula ##STR7## for the first fraction, representing theperfluoro-[α-3,6-dimethyl-1,4-dioxanyl-2-]-propionic acid fluoride,hitherto unknown.

794 g of this substance (1.67 mole) were then added dropwise to 160 mlof water while cooling with ice and stirring. Then the reaction mixturewas neutralized with an aqueous 20 percent by weight KOH solution andconcentrated in a rotation vaporator. The material thus predried waskept on a drying sheet in a vacuum oven for 24 hours at a temperature of100° C. under a pressure of 300 torrs, it was ground to fine particlesand heated again for the same period to 100° C. under a pressure of 0.1torr.

The dry material obtained was heated in a 2 liter round-bottomed flaskcoupled with two refrigerating traps, under a vacuum of from 5 to 1 torrfor a period of 30 hours at a temperature of from 200 to 225° C. Thecollected pyrolysate (628 g) was distilled by fractionation.

After a first fraction of 8 g (boiling point of from 97 to 103° C.), 584g (85.2% of the theory) ofperfluorinated-3,6-dimethyl-1,4-dioxanyl-2-vinyl ether having a boilingpoint of from 103 to 106° C. were obtained. A residue of 32 g did notdistill. ##STR8##

The infra-red and ¹⁹ F-NMR spectra back this structure. There appears astrong band at 9.65 μ in the infra-red spectrum.

Analysis

Calculated: C 23.4%; F 64.9 %.

Found: C 23.6%; F 65.2%.

EXAMPLE 2 Perfluoro-[α-(3,6-dimethyl-1,4-dioxanyl-2-oxy-propyl)-vinylethers]

From the second fraction formed during the preparation ofperfluoro-[α-3,6-dimethyl-1,4-dioxanyl-2-oxy]propionic acid fluorideaccording to Example 1, 67% by weight of a compound having a boilingrange of from 160 to 164° C. could be obtained by a second fractionateddistillation. The compound has the structure ##STR9## according to theelementary, infra-red and NMR spectroscopic analyses, beingperfluoro-[α-(3,6-dimethyl-1,4-dioxanyl-2-oxy-propoxy)]propionic acidfluoride.

Analysis

Calculated: C 22.4%; F 65.2%.

Found: C 22.3%; F 65.0%.

623 g (0.97 mole) of said acid fluoride were given dropwise to 100 ml ofwater while cooling with ice. Then the mixture obtained was neutralizedwith aqueous 10percent by weight KOH solution and concentrated in therotation evaporator. The predried material was then kept firstly for 15hours at a temperature of 100° C. under a pressure of 300 torrs and thenfurther dried for 62 hours at a temperature of 100° C. under a pressureof 0.1 torr. Then it was heated to 200° C. under a pressure of from 5 to0.1 torrs for 24 hours. The pyrolysis was performed according toExample 1. The distillation of the pyrolysate yielded 346 g (61.9% ofthe theory) of a vinyl ether having a boiling point of from 151 to 154°C. and a formula ##STR10## according to the analyses by NMR, IR andmass-spectrometry and to the elementary analysis.

Analysis: Calculated: C 22.9%; F 65.9%. Found: C 22.8%; F 65.5%.

EXAMPLE 3 Saturation of the terminal group of a hexafluoropropene oxidepolymer

100 g of a hexafluoropropene oxide polymer having a boiling point offrom 85 to 107° C./ 1 mm, an average molecular weight of about 1100 andthe formula ##STR11## wherein x + y = 4 (average value), was dissolvedin 70 ml of diglyme. 30 g of CsF and 200 g ofperfluoro-3,6-dimethyl-1,4-dioxanyl-2-vinyl ether were added and themixture was stirred for 55 hours at 60° C. The heavy phase was separatedand distilled. 120 g of a colorless liquid were obtained as mainfraction having a boiling point of from 93 to 138° C. under 0.09 torr,in which acid fluoride groups (5.32₈₂ ) could not be detected by IRspectroscopic analysis, whereas a band probably belonging to theβ-carbonyl ether group (C=O), appeared at 5.62μ.

The average molecular weight (about 1900) as well as the C/F analyses ofthe termal product did agree with the calculated composition C₃₆ F₆₆O₁₄.

What is claimed is:
 1. A process for preparing a perfluorinated vinylether of the general formula ##STR12## which comprises hydrolyzing anacid fluoride of the formula ##STR13## wherein n is 0 or 1, to form thecorresponding perfluorinated carboxylic acid, thereafter neutralizingthe acid and heating the resulting salt of the carboxylic acid to atemperature of 160° to 300° C. at a pressure of less than 10 torrs. 2.Process as claimed in claim 1, which comprises heating the carboxylicacid salt to a temperature of from 190 to 230° C.
 3. Process as claimedin claim 1, which comprises performing the heating of the carboxylicacid salt under reduced pressure.
 4. A process for preparing aperfluorinated vinyl ether of the general formula ##STR14## whichcomprises hydrolyzing an acid fluoride of the formula ##STR15## to formthe corresponding perfluorinated carboxylic acid, thereafterneutralizing the acid and heating the resulting salt of the carboxylicacid to a temperature of 160° to 300° C. at a pressure of less tha 10torrs.
 5. A process for preparing a perfluorinated vinyl ether of thegeneral formula ##STR16## which comprises hydrolyzing an acid fluorideof the formula ##STR17## to form the corresponding perfluorinatedcarboxylic acid, thereafter neutralizing the acid and heating theresulting salt of the carboxylic acid to a temperature of 160° to 300°C. at a pressure of less than 10 torrs.